Liquid crystal devices operate on the basic principle that due to the dielectric anisotropy of nematic, cholesteric and smectic liquid crystals, the average molecular axis, or director, can be oriented in the presence of an applied electric field. The coupling of non-ferroelectric liquid crystals to the applied field is a weak, second order interaction. In general, slow response times are characteristic of non-ferroelectric, non-chiral, liquid crystal optical devices.
Meyer et al. ("Ferroelectric Liquid Crystals", in Le Journal de Physique, Vol. 36, March, 1975, pp. L69-L71) showed that chiral C* or H*, smectic liquid crystals, could be ferroelectric, that is, possess a permanent electric dipole density, P. This permanent polarization, P, is perpendicular to the average orientation of the long axis of the molecules (denoted by the molecular director, n,) and generally contained within a plane parallel to the smectic layers. In these chiral smectic liquid crystals (CSLCs), the molecular director makes a temperature dependent angle, .PSI., with respect to the layer normal, z, as shown in FIGS. 1 and 2. In general, .PSI. ranges from 0.degree. to 45.degree.. The presence of the electric dipole provides a much stronger coupling to the applied electric field as compared to non-ferroelectric liquid crystals. Furthermore, the coupling, and hence aligning torque is about linear with applied field. The significance of this is that changing the sign of the applied electric field will change the direction of P in smectic C*, H*, A* and other chiral smectic phase liquid crystals.
N. A. Clark et al. in U.S. Pat. No. 4,367,924, realized a ferroelectric liquid crystal switching device by sandwiching a thin layer of a smectic C* (SmC*) liquid crystal between two glass plates coated with transparent electrodes. In this patent, they describe the surface-stabilized ferroelectric liquid crystal (SSFLC) device, which employs SmC* or SmH* liquid crystal phases in the so-called bookshelf geometry, otherwise designated the planar alignment, where the smectic layers are perpendicular to and the liquid crystal molecules are parallel to the glass plates which also contain the electrodes, as illustrated in FIG. 1 (see also N. A. Clark et al. U.S. Pat. No. 4,563,059 and N. A. Clark and S. T. Lagerwall in Applied Phys. Letts. (1980) 36:899 and S. T. Lagerwall and I. Dahl Mol. Cryst. Liq. Cryst. (1984) 114:151-187). SSFLC SmC* materials have been shown to be useful in a number of electro-optic device applications including switches, shutters, displays and spatial light modulators (SLM's). The advantages of planar aligned chiral smectic C,F,G,H, and I liquid crystal devices is their nearly three orders of magnitude increase in switching speeds over non-chiral liquid crystal devices and their intrinsic bistability, which has applications for optical memory units.
Tristable switching of a planar-aligned CSLC cell has been reported (I. Nishiyama et al. (1989) Jpn. J. App. Phy. 28:L2248; and A. D. I. Chandani et al. (1988) Jpn. J. App. Phy. 27:L729). The third state of such tristable cells has been linked with the presence of an antiferroelectric phase, designated SmCA*. This type of CSLC cell has been designated an antiferroelectric LC cell. CSLC materials which can exhibit this antiferroelectric effect have been reported by K. Furukawa et al. (1988) Ferroelectrics 85:63; M. Johno et al. (1989) Jpn. J. App. Phy. 28:L119 and Y. Suzuki et al. (1989) Liq. Cryst. 6:167.
Lagerwall et al. in U.S. Pat. No. 4,838,663, describe a non-tilted, non-ferroelectric, chiral smectic A* (SmA*) liquid crystal electro-optic switch. With planar-aligned, surface-stabilized SmA* material between substrate walls with no electric field applied (zero field state), n is parallel to z (i.e., .PSI.=0.degree.). The molecular director of the SmA* material exhibits rotation in a plane relative to z (.PSI..noteq.0.degree.) in response to an applied electric field due to the electroclinic effect (first described by S. Garaff and R. B. Meyer (1977) Phys. Rev. Letts. 38:848). These cells display an analog dependence of .PSI. with applied field to a maximum tilt angle .PSI..sub.MAX, which angle is an intrinsic property of the SmA* material. Materials having .PSI..sub.MAX ranging from about 6.degree. to 22.5.degree. have been observed (see also, Sharp, G. D. et al., (Opt. Lett. 15) (1990) pp. 523-525). The advantage of these planar-aligned SmA* cells is submicrosecond switching speeds and analog rotation of the optic axis.
L. A. Beresnev et-al., European Patent Application No. 309774, published 1989, has recently described a new type of chiral smectic ferroelectric liquid crystal cell called the distorted helix ferroelectric (DHF), liquid crystal cell. This type of device is similar to the planar-aligned chiral SmA* device of Lagerwall et al., except that it is not strongly surface-stabilized, so that the helix along the direction of the layer normal, z is not suppressed. Application of an applied electric field to the DHF cell perpendicular to z, partially orients the molecular directors by an angle .PSI. to z. The angle .PSI. is dependent on the size and magnitude of the field so the DHF device operates in an analog mode similar to a SmA* device. In a DHF device there is a change in the birefringence of the material as the molecules align, which does not occur in either the SSFLC SmC* or planar-aligned SmA* device. The DHF materials, such as Hoffmann-La Roche DHF 6300, having .PSI..sub.MAX as large as .+-.37.degree. have been described. The advantage of DHF switching devices over other FLC switching devices described above is the variable birefringence with applied voltage. This is similar to the operation of nematic liquid crystals, which also yield a variable birefringence with applied voltage. In contrast to nematic liquid crystals, the DHF molecular directors rotate by their full tilt angle within 40 .mu.sec, a significant advantage. Furthermore, the voltages required to rotate the optic axis are generally much lower than those required for SmA* and SmC* cells. An interesting feature of DHF devices is the coupling of the change in birefringence with the rotation of the optic axis as a function of applied voltage.
Z. M. Brodzeli et al. (1990) Technical Digest on SLM's and Their Applications 14:128 have reported fast electro-optic response (20 .mu.sec) in a homeotropically-aligned SmC* liquid crystal. In homeotropic alignment, the smectic layers of the liquid crystal are parallel to the surfaces of the substrate walls (see FIG. 2) and as in planar-aligned CSLCs, the molecular director makes an angle, .PSI., with the layer normal. In the optical modulator described by Brodzeli et al., the homeotropically-aligned SmC* material is positioned between substrate walls having deposited electrodes (the width of the cell was given as 17 .mu.m.) Polarized non-monochromatic light entering the device, propagating along the axis normal to the layers, was reported to be modulated in intensity by application of a voltage across the electrodes.
Phase modulation of optical signals is often accomplished by means of an electro-optic effect in which a change in index of refraction of a suitable material is achieved with the application of an electric field, for example, by the Pockels or Kerr effect (see, e.g., Yariv, A. and Yeh, P. Optical Waves in Crystals (1984) Wiley and Sons, N.Y.). While the Pockel's and Kerr effects are high speed effects, they require large voltages for bulk implementations in order to achieve very small electro-optic effects. A technique that has been used to improve the characteristics of electro-optic Pockel's and Kerr effect phase modulators, is to fold the optical path length using a Fabry-Perot etalon or resonator, which transforms the low amplitude input signal to an output optical intensity with high contrast.
A Fabry-Perot device consists of two plane parallel, highly reflecting surfaces, or mirrors, separated by a distance, L. When the mirrors are fixed at distance L, the device is called an etalon. When L can be varied the device is called an interferometer. A Fabry-Perot etalon operates on the principle of multiple interference of the waves reflected or transmitted by the mirrors. If L is a multiple of .pi., then the transmitted waves destructively interfere and the light incident upon the device is ideally totally reflected by the etalon. If L is a multiple of 2.pi., all the light is ideally transmitted by the etalon (assuming no absorption losses). If the etalon thickness is somewhere in between .pi. and 2.pi., then partial transmission or reflection occurs. If the optical thickness of the etalon can be changed, the etalon operates as a variable modulator.
Miller et al., U.S. Pat. No. 4,790,643, disclose an optically bistable device comprising a Fabry-Perot etalon containing an intracavity, optically non-linear, nematic liquid crystal material. The device provides an electro-optic bistable switch that is designed to modulate a monochromatic or coherent light source. Since the liquid crystal of this device is neither chiral or ferroelectric, the switching speed of this particular optical modulator is relatively slow.